Caution is demanded when connecting a photovoltaic array to an AC power grid via an inverter to avoid the occurrence of excessive currents through the circuitry components used and to avoid the occurrence of excessive voltages over the circuitry components used. The danger of overloading these components may, in principle, be eliminated by a higher voltage class or higher current class of the components. This approach normally, however, results in extreme cost increases and in power losses due to the lower efficiency of the higher voltage class components. Thus, methods and apparatuses are preferred that avoid high loads at the circuitry components upon connecting the photovoltaic array to the AC power grid.
This particularly applies in context with a development towards higher output voltages of photovoltaic arrays in order to be able to transfer higher electric power without a need for increasing the cable cross-section. Such higher output voltages of the photovoltaic arrays in normal operation go along with even higher open circuit voltages of the photovoltaic arrays. These open circuit voltages may readily exceed the permissible voltage across the inverter bridge of the respective inverter.
From DE 20 2006 001 063 U1 it is known to have a DC/DC converter, like for example a buck converter, preceding the inverter bridge of an inverter, the DC/DC converter providing a voltage adaptation between the photovoltaic array and the inverter. The buck converter, however, increases the complexity of the apparatus. It also causes additional power loss. DE 20 2006 001 063 U1 does not deal with connecting the photovoltaic array at high open circuit voltages to the inverter. It would be possible to avoid loading the inverter bridge with the full open circuit voltage of the photovoltaic array by dedicated operation of a buck converter preceding an inverter; however, the additional power loss caused by the buck converter during normal operation would remain, unless it was bridged in the current path by further means when the open circuit voltage has reduced to the output voltage that occurs during the normal operation of the photovoltaic array.
From JP 11312022 A it is known to protect the inverter bridge of an inverter from high open circuit voltages of a connected photovoltaic array by means of a voltage divider realized by a resistor arranged in one of the connection lines and a resistor connected in parallel to the inverter bridge. By shortening the resistor in the connection line and switching off the resistor connected in parallel to the inverter bridge, the voltage divider is disabled in normal operation, thus avoiding continued losses due to the resistors. The voltage divider involves an additional effort, particularly for switching it off in normal operation.
Inverters for feeding electric energy from photovoltaic arrays typically have a voltage control unit for a DC link in order to allow for the execution of a so-called Maximum Power Point (MPP)-tracking. In MPP-tracking the output voltage of the photovoltaic array is adjusted via the voltage of the DC link to a value at which the photovoltaic array yields a maximum electric power in view of actual operation conditions.
US 2009/0167097 A1 discloses a photovoltaic inverter interface between a photovoltaic array and an inverter. During a start-up process, a DC link of the inverter is pre-charged to the operating voltage of the inverter using soft start switch gear or the photovoltaic array. The inverter is then turned on while a contactor switch in one of the connection lines of the photovoltaic array is still open. Before the photovoltaic array is connected to the DC link of the inverter by closing the contactor switch, it is deliberately shorted by a shunt switch to reduce the output voltage from open circuit voltage to the output voltage under normal operation conditions.
There still is a need for a method of connecting a photovoltaic array to an AC power grid via an inverter, and for an apparatus feeding electric energy from a photovoltaic array into an AC power grid, the method and the inverter being able to cope with the problem of a high open circuit voltage of the photovoltaic array while at the same time keeping the technological complexity low.